A technique for supplying power to a load device using a non-contact system is known. As a product applying such a technique, charging systems of portable phones are generally used. Moreover, in recent years, as power feeding systems for electric vehicles, non-contact power feeding systems are at the stage of commercialization, and various standards have been determined.
As the non-contact power feeding system, there are various types such as an “electromagnetic induction type”, a “radio wave type”, and a “resonance type”. In addition, one kind attracting attention as a power feeding system for electronic vehicles and the like is the resonance type. FIG. 1 is a diagram showing the principle of a resonance type non-contact power feeding system, and the basic principle thereof has been developed and proven by MIT (Massachusetts Institute of Technology). In the resonance type non-contact power feeding system illustrated, a high-frequency power source and a power transmission loop (primary coil) are directly coupled, and a power reception loop (secondary coil) and a load are directly coupled. This system configures a resonance system in which power is transmitted contactlessly. Specifically, a power transmission side (primary side) device includes the high-frequency power source, the power transmission loop, and a primary resonance coil. A power reception side (secondary side) device includes a secondary resonance coil, the secondary coil, and the load (battery). In this system, as the power transmission side device and the power reception side device are subjected to magnetic field coupling (are electromagnetically coupled) by resonance, there is a possibility of about several kilowatts of power being transmitted for a relatively long distance. For example, there are research reports stating that power can be supplied to a space distant by about several meters with a high transmission efficiency (sometimes around 50%).
From such characteristics, wide research and development on the resonance type non-contact power feeding system have been made, and a technique for realizing an improvement in power transmission efficiency (for example, refer to PTL 1), a shield technique for allowing the system to function as an actual apparatus (for example, refer to PTL 2), and the like are disclosed.
Here, the necessity of the shield technique will be simply described. FIG. 2 shows a model when a basic model shown in FIG. 1 is actually mounted in a system. AC power is output from a high-frequency power source, and is supplied to a power transmission side resonance coil portion on a transmission line. By electromagnetic coupling that is intensified by the resonant action between the power transmission side resonance coil portion and a power reception side resonance coil portion, the AC power is transmitted to the power reception side resonance coil portion contactlessly. The AC power transmitted to the power reception side resonance coil portion is supplied to a rectifier on the transmission line. DC power converted from the AC power by the rectifier is supplied to a battery by the transmission line.
As such, in an actual system, a transmission path between a power source and a primary resonance portion and a transmission path between a secondary resonance portion and a rectifier are needed, and the transmission paths are also included in a resonance system. Therefore, electromagnetic coupling also occurs in the transmission paths (transmission wires). As a result, an electromagnetic field (radiated electromagnetic field) is generated from the transmission paths by induced current. The electromagnetic field radiated here is lost and the transmission efficiency is reduced. In addition, the electromagnetic field (radiated electromagnetic field) generated from the power transmission side resonance coil portion and the power reception side resonance coil portion is radiated into a space. This is also a loss, and the transmission efficiency is reduced.